Breathing aid apparatus in particular for treating sleep apnoea

ABSTRACT

A compressor driven by a motor sends to a nasal mask a breathable gas at a low positive relative pressure whereby the motor is controlled to maintain the pressure in the delivery pipe of the compressor substantially equal to a set point, independently of the inspiration and expiration of the patient, a computer receiving on an input a motor speed signal as a parameter representative of the respiratory activity of the patient and analyzing the motor speed variations whereby the computer will increase the pressure set point if necessary or reduces the pressure set point by a predetermined amount depending upon whether there is a hypopnoea or the absence thereof.

PRIOR APPLICATION

This application is a Continuation of U.S. patent application Ser. No.360,720 filed Dec. 12, 1994, now abandoned which is a 371 ofPCT/FR93/00547 filed Jun. 9, 1993.

FIELD OF THE INVENTION

The present invention relates to a breathing aid apparatus, inparticular for treating people which are prone to the disease called“sleep apnoea”. BACKGROUND OF THE INVENTION

Sleep apnoea syndrome (SAS) is the accumulation of signs as well astheir consequences due to the periodic interruption of respirationduring sleep. The re-establishment of respiration generally only occurswhen the person concerned wakes up. This phenomenon can occur severalhundred times per night, with interruptions of 10 seconds or more eachtime.

Three types of apnoea syndrome exist, each corresponding to a particularpathology.

The first type, which is the most common, is obstructive apnoea. Itresults from an obstruction of the upper respiratory tracts caused by acollapse of the tongue and the palate. The respiratory movementscontinue, but because of this obstruction, air can neither enter norleave the lungs.

The second type, which is rarer, is called “central apnoea”. It isproduced when the respiratory center of the brain no longer controlsrespiration. In the absence of a signal originating from the brain, therespiratory muscles do not function and air can neither enter nor leavethe lungs.

The third type is mixed apnoea which is a combination of the twoprevious types, the start of the apnoea being of central type.

In the case of obstructive apnoea and mixed apnoea, treatment bycontinuous positive pressure is the most commonly used. This techniqueconsists of permanently applying, via a nasal mask connected by a pipeto a pressure generating apparatus, a low positive relative pressure inthe upper respiratory tracts in order to avoid their obstruction. Thispressure prevents the tongue and palate from sticking together. Theresult is immediate: interrupted respiration is re-established, thelungs receive the oxygen they need and the person sleeps much better.

The optimum value of the pressure corresponds to the minimum allowingthe suppression of apnoeas and the oxygen desaturations which result inthe blood.

Determination of this optimum pressure is carried out in the laboratory,by subjecting the patient to a polygraph recording, and by progressivelyraising the level of pressure applied to the patient until thedisappearance of respiratory incidents.

The treatment described previously, which consists of applying aconstant pressure level to the patient throughout the night, has certaindeficiencies.

In fact, the frequency and extent of apnoeas vary during the nightaccording to the stage of sleep the patient is in. Also, they vary overtime as a function of the development of the condition of the patient(gain or loss of weight, absorption of alcohol before going to sleep . .. ) .

Therefore, the treatment pressure determined by the prescription is notnecessarily adequate subsequently. Now, control recordings cannot becarried out regularly, due to their cost and the significant burden onsleep laboratories, connected with the large number of patients to betreated.

In addition, the patient is subjected to an identical pressure allnight, whereas depending on the stages of his sleep, a lower pressuremay be sufficient, or a higher pressure may be necessary. Now, the lowerthe average pressure applied during the night is, the better thepatient's comfort will be and therefore his acceptance of the treatment,and the more the deleterious effects linked with too high a pressurewill be minimised.

SUMMARY OF THE INVENTION

The aim of the present invention is to propose a breathing aid apparatuswhich allows the treatment to be optimized as a function of theeffective needs of the patient at each stage of treatment.

According to the invention, the breathing aid apparatus, in particularfor treating sleep apnoea, comprising means of producing a flow ofbreathable gas under a low positive relative pressure, and means forleading this flow to a respiratory mask, is characterized in that inaddition the apparatus comprises means of acquiring a parameterrepresentative of the respiratory activity of the patient, and automaticadjustment means for increasing the pressure applied at least when therepresentative parameter is indicative of a hypopnoea, and for reducingthe applied pressure when the representative parameter is indicative ofnormal respiration over a predetermined time.

The term “hypopnoea” encompasses the phenomena of the totaldisappearance of respiration, and can also include certain phenomena ofpartial disappearance of respiration, due to a partial obstruction ofthe upper respiratory tracts.

Thanks to the invention, the treatment apparatus is no longer a simpleconstant pressure generator, but becomes an apparatus capable ofdetecting hypopnoeas and of adjusting the pressure level in order tosuppress the hypopnoeas.

In this way, thanks to the apparatus, each time a hypopnoea is detected,the pressure is increased, preferably by increments, until the hypopnoeaceases. When no hypopnoea has occurred for a defined period of time, thepressure is reduced by a predetermined value.

This process allows hypopnoeas to be put to an end while permanentlyminimizing the applied pressure.

Preferably, the pressure cannot go below a lower threshold defined bythe consultant and set on the apparatus, and of course it cannot exceedthe maximum value that the apparatus is capable of delivering, or amaximum value defined by the doctor.

Other characteristics and advantages of the invention will becomeapparent from the description below, with reference to thenon-limitative examples.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the attached drawings:

FIG. 1 is a diagram of an apparatus according to the invention;

FIG. 2 is a flow chart for the operation of the computer of FIG. 1;

FIGS. 3 and 4 are diagrams similar to FIG. 1 but relating to two otherembodiments; and

FIG. 5 is a flow chart of the operation of the computer of theembodiment of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus represented in FIG. 1 comprises a compressor 1 capable ofproducing through its delivery pipe 2 a breathable gas at a positiverelative pressure, i.e. measured relative to atmospheric pressure, whichdepends on the rotational speed of the drive motor 3. In anon-represented manner, the compressor 1 is of a type which produces thepositive relative pressure by a turbine for propelling breathable gas.The delivery pipe 2 is connected to a nasal mask 4 by a flexible tube 6.The nasal mask 4 is intended to be applied to the patient's face, forexample by means of a strap. The mask 4 includes an opening 7 allowingthe patient to expire despite the flow in the opposite direction comingfrom the compressor 1.

A comparator 8 permanently compares the pressure P_(m) detected in thedelivery pipe 2 of the compressor 1 by a pressure detector 9 with apressure set point P_(c) applied to the other input 11 of the comparator8. As a function of the result of the comparison, the comparator 8supplies at its output 12 a signal applied to a motor control device 13to reduce the rotational speed of the motor 3 when the pressure measuredby the detector 9 is greater than the pressure set point, and toincrease the rotational speed of the motor 3 and therefore the pressureat the delivery pipe 2 when the pressure measured by the detector 9 islower than the pressure set point.

In this way, the pressure at the delivery pipe 2 and therefore in thenasal mask 4, is approximately the same during the inspiration phasesand during the expiration phases of the patient.

During the inspiration phases, a relative low pressure tends to becreated at the delivery pipe 2 of the compressor 1, and maintaining thepressure at the set point value requires an increase in the rotationalspeed of the motor 3.

On the other hand, during the expiration phases of the patient, anexcess pressure tends to be created at the delivery pipe 2, andmaintaining the pressure at the set point value requires a decrease inthe rotational speed of the motor 3.

Consequently, when the respiration of the patient is normal, therotational speed of the motor 3 follows a periodical curve.

According to the embodiment in FIG. 1, a signal representative of therotational speed of the motor 3 is applied by the control device 13 tothe input 14 of a computer 16 whose function is to analyze the curve ofthe speed of the motor 3 as a parameter representative of therespiratory activity of the patient, and to modify the pressure setpoint P_(c) applied to the input 11 of the comparator 8 as a function ofthe result of this analysis.

In a general fashion, when the analysis of the curve of the rotationalspeed of the motor reveals a hypopnoea situation, the computer 16increases the pressure set point.

On the other hand, if the analysis of the curve of the speed of themotor reveals an absence of hypopnoea for a certain predetermined periodof time, the computer reduces by a predetermined amount the pressure setpoint.

The computer 16 is connected to a manual control 17 allowing the minimumpressure set point P_(min) authorized by the doctor for each patient tobe adjusted.

There will now be described with reference to FIG. 2, the flow chartaccording to which, essentially, the computer 16 is programmed.

In what follows, by “hypopnoea ”is meant the symptom consisting eitherof an abnormal lowering (for example by 50%) of the respiratoryactivity, or the symptom of total apnoea consisting of the completedisappearance of respiratory activity.

At the start, the pressure set point P_(c) is chosen to be equal toP_(min), i.e. the minimum pressure set point chosen using the manualcontrol 17 (stage 18).

In stage 19, the values An-8, An-7, ..., An-1 of the amplitude of themotor speed variation during the eight respiratory cycles before the onewhich is currently being analyzed, are arbitrarily set equal to a valueA0 which is relatively low.

Then, in stage 21, the average of the amplitudes of the eight previouscycles (average M) is calculated and two thresholds S1 and S2 arecalculated with for example:

S1=0.8M

S2=0.7M

In stage 22, the extreme values of the rotational speed of the motor aresought.

In order to do this, the rotational speed of the motor at each executioncycle of the program is stored in memory. A maximum or minimum is onlyvalidated if the speed has then varied sufficiently so as to be backfrom this maximum or minimum by a value at least equal to threshold S2.

In other words, as the threshold S2 is greater than half of the averageof the previous amplitudes, a given extreme value will only be processedif the speed again then reaches a value beyond that of the average ofthe speeds. In particular, if respiration stops (total apnoea), thespeed of the motor assumes its average value and the previous extremevalue is not validated. More generally, if an amplitude lower thanthreshold S2 tends to become established, it will no longer be possibleto validate the extreme values.

After a period of time T1 equal for example to 10 seconds, this isdetected in the following test 23. In the absence of an extreme valuefor 10 seconds, one follows the path “detection of strong hypopnoea” 24of the flow chart, in which the four amplitudes An-8 . . . An-5 whichare the oldest values still in memory are reduced to the relatively lowvalue of A0. The aim of this is to reduce the thresholds S1 and S2 forthe next calculation cycle so as to make the resumption of respiratoryactivity easier to detect.

Returning to test 23, if an extreme value was found within the 10previous seconds and if this extreme value is the same as that alreadyprocessed during the previous calculation cycle, one returns to stage 22in order to search for extreme values.

If, on the other hand, the extreme value is new, one passes via stage 26for calculating the new amplitude An, then, stage 27, storing in memorythe amplitude An while simultaneously deleting the oldest amplitude inmemory An-8.

In stage 28, the newly-calculated amplitude An is compared with thelargest S1 of the two thresholds.

If the newly-calculated amplitude An is greater than threshold S1, onefollows normal respiration path 29 which will be described further on.

In the opposite case, i.e. if the amplitude is between thresholds S1 andS2, it is considered that a weak hypopnoea 31 exists.

Whether strong hypopnoea 24 or weak hypopnoea 31 has been recorded, atest 32 is carried out in order to determine whether there was already ahypopnoea during the previous 30 seconds. If the result is negative anumber MAP is reset to zero. MAP corresponds to the total increase inpressure in the previous 30 seconds.

If, on the other hand, there was hypopnoea during the previous 30seconds, the MAP number is not reset to zero.

The following stage 33 consists of adding a relatively high increment tothe MAP number if strong hypopnoea was detected, and a relatively lowincrement if weak hypopnoea was detected. Then, in stage 34, a test iscarried out to establish whether the MAP number is greater than 6 cm ofwater (6hP_(a)). If the result is negative, stage 36, an increment X,being high or low depending on the strength of the hypopnoea, is addedto the pressure set point P_(c). If, on the other hand, MAP exceeds 6,the pressure set point P_(c) is only increased to the extent that thetotal increase in the previous 30 seconds is equal to 6 (stage 37).

The aim of this is to avoid increasing the pressure excessively to treata single hypopnoea: if an increase of more than 6 cm of water isnecessary to treat a hypopnoea, it is because there is some anomaly andit would be better to wake the patient up.

Then, the new pressure set point is applied to the comparator 8 in FIG.1 on the condition that it does not exceed the maximum pressure setpoint P_(max). If the pressure P_(c) exceeds P_(max) the set pointapplied to the comparator 8 is equal to P_(max) (stage 38). One is thenreturned to stage 21 in which the thresholds are calculated. If thestrong or weak hypopnoea which was detected during the previous cycle isstill not alleviated, the pressure set point will be increased by a newincrement and so on until the total pressure increase MAP within 30seconds reaches 6 cm of water or until the hypopnoea is alleviated.

In this way, the amplitude is compared to two different thresholds, oneto detect strong hypopnoeas, including the total hypopnoeas, and toapply a relatively swift increase in the pressure set point, the otherto detect weak hypopnoeas, resulting from a partial obstruction of theupper respiratory tract, and to apply a clearly milder increase inpressure.

One of the important features of the invention consists of analyzing theparameter representative of respiratory activity (the speed of the motor3) not by comparison with absolute thresholds, but by comparison withthe respiratory activity which has just preceded the respiratoryanomaly. In fact, it has been noted that respiratory activity variesgreatly during sleep, to the extent that an activity which would beconsidered normal during a certain phase of sleep can correspond to ahypopnoea in another phase of sleep.

Returning to path 29 of the flow chart, this leads to a test 39 fordetermining whether a time T has passed without detecting a hypopnoea.If the result is negative, one returns to stage 21 in which thethresholds are calculated.

If, on the other hand, a time T2, for example equal to 30 minutes, haspassed without a hypopnoea, the pressure set point is reduced by, forexample, 2 cm of water. In this way one provides an opportunity to bringthe pressure applied to the patient to a lower value if this ispossible.

However, if the new pressure set point thus became lower than theminimum pressure as set with the manual control 17 of FIG. 1, thepressure set point is simply reset equal to the minimum pressure set.Then, once again, one is returned to stage 21 in which the thresholdsare calculated.

In the example represented in FIG. 3, which will only be described withregard to its differences relative to that of FIG. 1, a flow ratedetector 41 is placed on the delivery pipe 2 of the compressor 1 whosesignal is sent to an input 42 of the computer. On the other hand thecomputer no longer receives a signal corresponding to the rotationalspeed of the motor. It is now the flow rate signal provided by thedetector 41 which provides the computer with the parameterrepresentative of the respiratory activity. When the patient inspires,the flow rate detector 41 reveals a higher flow rate than when thepatient expires. In other words, the variations in flow rate work in theopposite sense to those of the speed of the motor 3. Apart from that,nothing is changed, and the flow chart of FIG. 2 is valid for theembodiment of FIG. 3, with the exception that in stage 22 in which theextreme values are sought, the word “speed” must be replaced by thewords “flow rate”.

The example of FIG. 4 corresponds to a simplified version.

In this example, which will only be described with regard to itsdifferences relative to its differences relative to that of FIG. 1,there is no pressure regulation at the delivery pipe 2, i.e., apart fromsituations of apnoea or hypopnoea, the motor 3 rotates at the same speedwhether the patient inspires or expires. The pressure at the deliverypipe 2 is therefore relatively low when the patient inspires andrelatively high when he expires. Therefore, the pressure at the deliverypipe 2 constitutes a parameter representative of the respiratoryactivity and it is, as such, detected by the pressure sensor 9. Thecomputer 16, which receives the pressure signal 9 on an input 43,analyzes the pressure curve and provides the control device 13 of themotor 3 with a signal for increasing the speed of the motor 3 when thevariations in pressure indicate a, situation of hypopnoea, and fordecreasing the speed of the motor 3 when any situation of hypopnoea hasnot been alleviated within a predetermined period of time, for example30 minutes.

FIG. 5 represents a schematic flow chart according to which the computer17 of FIG. 4 can be programmed.

At the start, the speed V of the motor is adjusted to a value V_(min)(stage 44) set with a manual control 46 (FIG. 4).

Then one passes to stage 47 in which hypopnoeas are detected accordingto the amplitude of the variations in pressure. This stage cancorrespond to stages 21 and 22 of FIG. 2, except that it is then appliedto the pressure instead of being applied to the speed of the motor. Inthe absence of hypopnoea, one passes via path 48 in which the speed ofthe motor is reduced by a predetermined value n′ if a time T2, forexample 30 minutes, has passed without hypopnoea, without howeverlowering the speed to a value which is less than the set speed V_(min).

In the case of a hypopnoea being detected during a period of timegreater than or equal to a value T₁, of for example 10 seconds, thespeed V is incremented by a predetermined value n, without howeverallowing the speed to exceed a value V_(max).

Consequently, in this simplified example, only a single degree ofintensity of hypopnoea is distinguished and when the hypopnoea isdetected, one and the same mode of action is envisaged in every case,i.e. an incrementation of the speed of the motor according to onepredetermined step and one only.

Of course, the invention is not limited to the examples as described andrepresented.

In the computers of the embodiments according to FIGS. 1 and 3 a programcould be envisaged which distinguishes only one type of hypopnoea, or onthe other hand, the embodiment according to FIG. 4 could be equippedwith a program which processes in a different way the weak hypopnoeasand the strong hypopnoeas as was described with reference to FIG. 2.

What is claimed is:
 1. Breathing aid apparatus, comprising: means forproducing a flow of breathable gas to a patient having respiratoryactivity; means for controlling pressure of the flow of the breathablegas; means for calculating an amplitude of variation indicative of therespiratory activity of a patient, further including means forcalculating the amplitude of variation as a function of a variablemeasured from the means for producing a flow of breathable gas;detecting means for determining the presence of a hypopnoea from ananalysis of the amplitude of variation; and adjustment means forincreasing the pressure of the flow of breathable gas when saiddetecting means determines the presence of hypopnoea.
 2. The breathingaid apparatus of claim 1, wherein said adjustment means for increasingthe pressure includes means for adjusting the speed of operation of amotor driving a compressor adapted to produce the flow of breathablegas.
 3. The breathing aid apparatus of claim 1, wherein said means forproducing a flow of breathable gas includes a drive motor operablyconnected to a compressor.
 4. The breathing aid apparatus of claim 3,wherein said means for controlling pressure of the flow of breathablegas includes a motor control operably connected to the drive motor, apressure detector and a comparator operably connected to the motorcontrol.
 5. The breathing aid apparatus of claim 4, wherein said meansfor calculating an amplitude of variation includes a signal from themotor control indicative of the rotational speed of the drive motor. 6.The breathing aid apparatus of claim 5, wherein said detecting meansincludes means for comparing the present amplitude of variationindicative of the respiratory activity of the patient with at least onethreshold value calculated from at least one previous amplitude ofvariation of indicative of the respiratory activity of the patient,wherein said adjustment means increases the pressure of the flow ofbreathable gas when the amplitude of variation is lower than thethreshold value.
 7. The breathing aid apparatus of claim 6, wherein thethreshold value is calculated from an average amplitude of variationindicative of the respiratory activity of the patient calculated from atleast three previous variation periods.
 8. The breathing aid apparatusof claim 5, wherein said detecting means includes means for comparingthe amplitude of variation with a validation threshold, wherein saidadjustment means increases the pressure of the flow of breathable gaswhen the amplitude of variation remains below the validation thresholdfor a predetermined period of time.
 9. The breathing aid apparatus ofclaim 1, wherein said detecting means includes means for comparing theamplitude of variation with a first threshold for a weak hypopnoea and asecond threshold for a strong hypopnoea, wherein said adjustment meansincreases the pressure of the flow of breathable gas by a firstincremental adjustment when the amplitude of variation is greater thanthe second threshold for strong hypopnoea, and by a second incrementaladjustment when the amplitude is between the first and secondthresholds, such that the first incremental adjustment is greater thatthe second incremental adjustment.
 10. The breathing aid apparatus ofclaim 1, wherein said adjustment means includes means for reducing thepressure of the flow of breathable gas when said detecting meansdetermines the lack of a hypopnoea over a predetermined time. 11.Breathing aid apparatus, comprising: a compressor having a drive motorand configured to produce a flow of breathable gas to a patient; apressure detector in fluid communication with an outlet of saidcompressor; a comparator having a first input, a second input and anoutput, wherein the pressure detector generates a first signal connectedto the first input; a motor control operably connected to the drivemotor of said compressor and which generates a second signal indicativeof the rotational speed of the drive motor, wherein the motor controlaccepts the output of the comparator; and a computer configured toaccept the second signal from the motor control, the computer furtherconfigured to calculate an amplitude of variation based on the secondsignal and to detect the presence of a hypopnoea, wherein the computergenerates a pressure set point connected to the second input to thecomparator, such that the set point is calculated to increase thepressure of the flow of breathable gas when the amplitude of variationis indicative of a hypopnoea.
 12. The breathing aid apparatus of claim11, wherein said computer is further configured to compare the amplitudeof variation with at least one threshold value calculated from at leastone previous variation period, wherein said computer increases thepressure set point when the amplitude of variation is lower than thethreshold value.
 13. The breathing aid apparatus of claim 12, whereinthe threshold value is calculated from an average amplitude valuecalculated from at least three previous variation periods.
 14. Thebreathing aid apparatus of claim 11, wherein said computer is furtherconfigured to compare the amplitude of variation a validation threshold,wherein said computer increases the pressure set point when theamplitude of variation remains below the validation threshold for apredetermined period of time.
 15. The breathing aid apparatus of claim11, wherein said computer is further configured to compare the amplitudeof variation with a first threshold for a weak hypopnoea and a secondthreshold for a strong hypopnoea, wherein said computer increases thepressure set point by a first incremental adjustment when the amplitudeof variation is greater than the second threshold for strong hypopnoea,and by a second incremental adjustment when the amplitude is between thefirst and second thresholds, such that the first incremental adjustmentis greater that the second incremental adjustment.
 16. The breathing aidapparatus of claim 11, wherein said computer is further configured toreduce the pressure of the flow of breathable gas when the amplitude ofvariation is indicative of lack of a hypopnoea over a predeterminedtime.
 17. A method for treating sleep apnoea with a breathing aidapparatus, comprising: producing a flow of breathable gas to a patienthaving respiratory activity; controlling pressure of the flow of thebreathable gas; calculating an amplitude of variation indicative of therespiratory activity of the patient, further including calculating theamplitude of variation as a function of a variable measured from a meansfor producing a flow of breathable gas; and increasing the pressure ofthe flow of breathable gas when the amplitude of variation is indicativeof a hypopnoea.
 18. The method of claim 17, further including the stepsof comparing the amplitude of variation with at least one thresholdvalue calculated from at least one previous variation period, andincreasing the pressure set point when the amplitude of variation islower than the threshold value.
 19. The method of claim 11, wherein thethreshold value is calculated from an average amplitude value calculatedfrom at least three previous variation periods.
 20. The method of claim17, further including the steps of comparing the amplitude of variationwith a validation threshold, and increasing the pressure set point whenthe amplitude of variation remains below the validation threshold for apredetermined period of time.
 21. The method of claim 17, furtherincluding the steps of comparing the amplitude of variation with a firstthreshold for a weak hypopnoea and a second threshold for a stronghypopnoea, and increasing the pressure set point by a first incrementaladjustment when the amplitude of variation is greater than the secondthreshold for strong hypopnoea, and by a second incremental adjustmentwhen the amplitude is between the first and second thresholds, such thatthe first incremental adjustment is greater that the second incrementaladjustment.
 22. The method of claim 17, further including the step ofreducing the pressure of the flow of breathable gas when the amplitudeof variation is indicative of lack of a hypopnoea over a predeterminedtime.
 23. Breathing aid apparatus, comprising: means for producing aflow of breathable gas to a patient having respiratory activity; meansfor controlling pressure of the flow of the breathable gas; means forcalculating an amplitude of variation indicative of the respiratoryactivity of a patient, further including means for calculating theamplitude of variation as a function of a variable-measured from themeans for producing a flow of breathable gas; detecting means fordetermining the presence of a hypopnoea from an analysis of theamplitude of variation; and adjustment means for increasing the pressureof the flow of breathable gas when detecting means determines thepresence of a hypopnoea, and for decreasing the pressure of the flow ofbreathable gas when said detecting means determines the presence ofnormal breathing.
 24. Breathing aid apparatus, comprising: a compressorhaving a drive motor and configured to produce a flow of breathable gasto a patient; a pressure detector in fluid communication with an outletof said compressor; a comparator having a first input, a second inputand an output, wherein the pressure detector generates a first signalconnected to the first input; a motor control operably connected to thedrive motor of said compressor and which generates a second signalindicative of the rotational speed of the drive motor, wherein the motorcontrol accepts the output of the comparator; and a computer configuredto accept the second signal from the motor control, the computer furtherconfigured to calculate an amplitude of variation based on the secondsignal and to detect the presence of a hypopnoea, wherein the computergenerates a pressure set point connected to the second input to thecomparator, such that the set point is calculated to increase thepressure of the flow of breathable gas when the amplitude of variationis indicative of a hypopnoea, and to decrease the pressure of the flowof breathable gas when the amplitude of variation is indicative ofnormal breathing.